Increasing the magnetic sensitivity of liquid crystals by rod-like magnetic nanoparticles P. Kopˇcansk´y1, N. Tomaˇsoviˇcov´a1, T.T´oth-Katona2, N. E´ber2, M. Timko1, V. Z´aviˇsov´a1, J. Majoroˇsov´a1, M. Rajnˇak1, J. Jadzyn3, X. Chaud4 1Institute of Experimental Physics, Slovak Academy of Sciences, Watsonova 47, 04001 Koˇsice, Slovakia 4 2Institute for Solid State Physics and Optics, 1 Wigner Research Centre for Physics, Hungarian Academy of Sciences, 0 H-1525 Budapest, P.O.Box 49, Hungary 2 3Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, 60179 Poznan, Poland n a 4High Magnetic Field Laboratory, CNRS, 25 Avenue des Martyrs, J Grenoble, France 8 ] MagneticFr´eedericksztransitionwasstudiedinferronematicsbasedonthenematic t f liquid crystal 4-(trans-4’-n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT). 6CHBT o was doped with rod-like magnetic particles of different size and volume concentration. s The volume concentrations of magnetic particles in the prepared ferronematics were t. φ1=10−4andφ2=10−3. Thestructuralchangeswereobservedbycapacitancemeasure- a mentsthatdemonstrateasignificantinfluenceoftheconcentration,theshapeanisotropy, m and/orthesizeofthemagneticparticlesonthemagneticresponseoftheseferronematics. - Keywords: ferronematics; liquid crystals; magnetic particles; phase transi- d tion, n o c 1. Introduction Liquid crystalline phases occur as additional, thermo- [ dynamically stable states of matter between the liquid state and the crystal state 1 in some materials. They can be characterized by a long-range orientational order v of the molecules and, as a consequence, by an anisotropy in their physical prop- 5 erties. Liquid crystals can be oriented under electric or magnetic fields due to the 9 6 anisotropy of dielectric permittivity or diamagnetic susceptibility [1]. 1 One of the most important findings related to controlling liquid crystals by . externalfieldswasthethresholdbehaviourinthereorientationalresponseofliquid 1 0 crystals – an effect described by V.K. Fr´eedericksz [2], and named after him as 4 ”Fr´eedericksztransition”. The dielectric permittivity anisotropyof liquid crystals 1 is in general relatively large; thus driving voltages of the order of a few volts : v are sufficient to control the orientational response. Therefore, most of the liquid i crystaldevicesaredrivenbyelectricfield. Ontheotherhand,becauseofthesmall X −7 valueoftheanisotropyofthediamagneticsusceptibility(χa ∼10 ),themagnetic r a fieldHnecessarytoalignliquidcrystalshavetoreachratherlargevalues(B=µ0H ∼ 1T), and therefore, liquid crystal applications using magnetic fields are rather limited. Consequently, the increase of the magnetic sensitivity of liquid crystals is an important challenge, which can potentially broaden the area of applications and may offer an opportunity to develop new devices. Brochard and de Gennes [3] first suggested the idea that could increase the magnetic sensitivity of liquid crystals theoretically. According to them, colloidal systems called ”ferronematics”, consisting of nematic liquid crystals doped with magnetic nanoparticles in small concentrations, should respond to low magnetic fields of the order of tens of Gauss. Such small magnetic fields cannot affect the undoped nematics, however,they may be sufficiently strong to align or rotate the magnetic moments of the particles inside the ferronematic suspensions according tothepredictions. Thisrealignmentorrotationeffectcouldthenbetransferredto the host nematic through the coupling between the nanoparticles and the liquid crystal molecules. One has to note here that the realignment of the nematic host was assumed to be entirely determined by the ferromagnetic properties of nanoparticles(notaffectedbytheintrinsicdiamagneticpropertiesofthenematic), since the theory [3] predicted a rigid anchoring with n k m, where n is the unit vectorofthe preferreddirectionofthe nematic molecules(director), andm is the unit vector of the magnetic the moment of the magnetic particles. The first experimental realization of ferronematic materials was carried out by Chen and Amer [4]. Later experiments on some other ferronematics have indicated that besides the predicted n k m condition, the case of n ⊥ m is also possible. To bridge this gap between the theory and experiments, Burylov and Raikher modified the theoretical description by considering a finite value of the surfacedensityofanchoringenergyW atthenematic–magneticparticleboundary [5, 6, 7, 8]. The finite value of W, as well as the parameter ω= Wd/2K (d is the mean diameter of the magnetic particles and K is an orientational-elastic Frank modulus), characterize the type of anchoring of nematic molecules on magnetic particle’s surface. For ω >> 1 the anchoring is rigid, while the soft anchoring is characterized by ω ≤ 1 which (unlike the rigid anchoring) permits both types of boundary conditions, n k m and n ⊥ m. So far the magnetic nanoparticles have established their wide range of appli- cations. The properties of magnetic nanoparticles significantly depend on their size, shape and structure. Controlling the shape and size of nanoparticles is one of the ultimate challenges in modern material research. These magnetic particles can be made so small that each particle becomes a single magnetic domain, ex- hibiting abnormal magnetic properties, known as superparamagnetism. Doping liquid crystals with low volume concentration of nanoparticles has been shown to be a promising method to modify the properties of liquid crystals. At such a low amount, nanosized particles do not disturb significantly the liquid crystalline order, hence producing a macroscopically homogeneous structure. However, the particles canshare their properties with the liquid crystalhost, enhancing the ex- isting properties, or introducing some new properties for the composite mixtures. 2. Experiment Two types of magnetic rod-like particles were prepared throughhydrolysisofFeCl3 andFeSO4 solutions(molarratio2:1)containingurea. To prepare the larger rod-like particles (sample A) 0.6756 g of FeCl3 · 6H2O, 0.3426 g FeSO4 · 7H2O and 0.60 g (NH2)2CO were dissolved in 10 ml of purified, deoxygenated water. The product was added to a flask with reflux condenser, ◦ which has been kept at 90-95 C for 12 h, and then cooled to room temperature [9]. After the synthesis particles were coated with oleic acid as a stabilizer. In the synthesis of the smaller rod-like particles (sample B), at first, the stabilizer (oleic acid) was ultrasonically dispersed in water to form homogenous micelles. Then, FeCl3 · 6H2O, FeSO4 · 7H2O were dissolved in the above solution. This mixture was added to a flask with reflux condenser and has been heated in water ◦ bath for 12 hours at 90-95 C during which a dark precipitate has been formed. The sample has been cleaned several times by purified and deoxygenated water, ◦ and then it has been dried under low pressure at 50 C for 3 hours [10]. Figure 1 showstransmissionelectronmicroscopicimagesofthepreparedmagneticparticles. The diameter of the larger rod-like particles (sample A) was dA=(18±3) nm and their mean length LA=(400 ± 52) nm. The mean diameter and length of the smaller rod-like particles (sample B) were dB=(10±1) nm and LB=(50±9) nm, respectively. Consequently,the meanshape anisotropiesofthe nanoparticleswere 2 approximately LA/dA ≈22 and LB/dB ≈5 for the two types of samples. (a) (b) Figure 1: TEM image of (a) the larger rod-like particles with mean diameter of 18 nm and mean length of 400 nm; (b) the smaller rod-like particles with mean diameter of 10 nm and mean length of 50 nm. Theferronematicsampleswerebasedonthethermotropicnematic4-(trans-4’- n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT), which was synthetized and purified at the Institute of Chemistry, Military Technical University, Warsaw, Poland. 6CHBT is a low-temperature-melting enantiotropic liquid crystal with high chemical stability [11]. The phase transition temperature from the isotropic ◦ liquid to the nematic phase has been found at 42.6 C. The doping was done by adding nanoparticles to the liquid crystal in the isotropic phase under continuous stirring. The structuraltransitions in the preparedsamples were monitored by capac- itance measurements in a capacitor made of ITO-coated glass electrodes. The capacitor with the electrode area of approximately 1 cm × 1 cm was placed into ◦ a regulated thermostat system, the temperature of which was stabilized at 35 C. Thedistancebetweentheelectrodes(samplethickness)wasD=5µm. Thecapac- itance was measured at the frequency of 1 kHz by the high precision capacitance bridge Andeen Hagerling. In the experiment the liquid crystal had a planar initial alignment, i.e. the directorwasparalleltothecapacitorelectrodes,andthemagneticfieldwasapplied perpendicular to them (see Fig. 2). The dependence of the measured capacitance on the external field reflects the reorientation of the nematic molecules. 3. Results Results presented in work [12] showed that doping with mag- netic particles shaped similarly to the molecules of the host liquid crystal is more effectiveandthusoffersbetterperspectivesforferronematicsinapplicationswhere a magnetic field is necessary to controlthe orientation of the liquid crystal. With theaimtostudytheinfluence ofthe sizeoftheparticlesonthemagneticresponse n m B Figure 2: Cross section of a planar cell with initially parallel n and m. 3 1.0 6CHBT -4 sample A 1=10 0.8 sample A 2=10-3 C)0 sample B 1=10-4 C-max 0.6 sample B 2=10-3 C)/(0 C- 0.4 ( 0.2 0.0 0 1 2 3 4 5 6 7 8 (a) B(T) 0.0007 6CHBT 0.0006 sample A 1=10-4 -3 0.0005 sample A 2=10-4 C)/(C-C)0max000..00000034 ssaammppllee BB 12==1100-3 C- ( 0.0002 0.0001 0.0000 0.0 0.1 0.2 0.3 0.4 0.5 (b) B(T) Figure 3: Reduced capacitance versus magnetic field (a) for pure 6CHBT and for 6CHBT doped with different rod-like particles and with different volume concen- trations of magnetic particles; (b) blowup of the low magnetic field region for the same data. twokindsofrod-likemagneticparticles(sampleAandsampleB)werepreparedas describedabove. BothferronematicsAandBwerebasedonthenematic6CHBT, andweredopedintwo differentvolumeconcentrationsφ1 = 10−4 andφ2 =10−3. Figure 3(a) shows the magnetic Fr´eedericksz transition in pure 6CHBT and in ferronematicsdoped with larger(A) and smaller (B) rod-like particles for both volume concentrations. It demonstrates that the critical magnetic field Bc of the Fr´eedericksz transition, i.e. the magnetic field that initiates the reorientation of the director toward its direction, is shifted to lower values with increasing the volume concentration, and that Bc is lower for larger particles than for smaller ones at a givenφ. For allsamples the critical magnetic field was determined from the dependence of (C −C0)/(Cmax −C0) versus B, where C, C0 and Cmax are the capacitances at a given magnetic field, at B = 0, and at the maximum value ofB,respectively. Bc wasdeterminedbylinearextrapolationofdatainFig.3(a). The obtained critical value of the magnetic field for pure 6CHBT is 2.63 T. In ferronematics Bc is lower, and the values obtained for various samples are listed in Table 1. The reduction of Bc becomes larger if the concentration is increased (in case of the same nanoparticle), as well as, if the nanoparticle is larger (at the same concentration). Observations of the structural transitions in ferronematics in external field 4 can be used for the determination of the type of anchoring of nematic molecules onthesurfacesofmagneticparticlesaswellasthesurfacedensityoftheanchoring energy W atthe nematic – magnetic particle boundary. By means of the Burylov and Raikher’s expression for the free energy of the ferronematic [7] the formula for the critical magnetic field is 2 2 2µ0Wφ B −B = , (1) c cFN χad where Bc andBcFN arethe criticalfields for the magnetic Fr´eedericksztransition of the pure liquid crystal and the ferronematic, respectively, d is the ”character- istic size” of the particles (the mean diameter), φ is the volume concentration of magneticparticlesintheliquidcrystal,µ0 isthepermeabilityofvacuumandχa is the anisotropy of the diamagnetic susceptibility of the liquid crystal (for 6CHBT χa = 4.805x10−7 at 35◦C). The calculatedvalues ofW andthe values ofparameterω aresummarizedin Table 1. ω has been calculated using the same K1 = 6.71 pN elastic constant for allferronematicsasforthe pure6CHBT.Inallcasesω <1thatcharacterizessoft anchoring of the nematic molecules on the surface of magnetic particles. sample BcFN (T) W (Nm−1) ω sample A φ1 2.39 4.14×10−5 0.055 sample A φ2 2.12 8.34×10−6 0.011 sample B φ1 2.52 1.08×10−5 0.008 sample B φ2 2.25 3.54×10−6 0.003 Table1: Criticalmagneticfieldsmeasuredinthe ferronematicsandthecalculated values of the surface density of the anchoring energy W, and of the parameter ω. InrecentworksbyPodoliaketal. [13],andBuluyetal. [14]bothexperimen- tal and theoretical investigations have been reported about the optical response ofsuspensionsofferromagneticnanoparticlesinnematicliquidcrystalsonthe im- posed magnetic field. The authors have measured a linear optical response in fer- ronematicsatverylowmagneticfields(farbelowthethresholdoftheFr´eedericksz transition). A similar effect was also observed in our dielectric measurements in samples dopedwithrod-likeparticlesasitisdemonstratedinFig.3(b). Thefigureprovides a clear evidence for a nearly linear magnetic field dependence of the capacitance in the low magnetic field region. 4. Conclusion Wehavedemonstratedthatboththethresholdofthemag- neticFr´eedericksztransitionandthedielectricresponsetolowmagneticfields(far below the Fr´eedericksz transition) depend not only on the volume concentration of the magnetic particles, but also on the size of the particles. According to the results, the larger is the particle, the bigger are the effects (larger decrease of the thresholdof the Fr´eedericksztransition, andmore pronouncedlinear response to low magnetic fields). Since in our experiments the larger particles have also a larger aspect ratio L/d, further experiments are needed to clarify whether the volume size, the linear size or the shape anisotropy(i.e. L/d) influences primarily the magnitude of the effects. The other challenging task is to explain the linear dielectric response to low magnetic fields. To our present understanding, within the framework of the 5 BurylovandRaikher’scontinuumtheory[5,6,7,8],boththemagneticmomentof magneticparticlem andthepresenceofaninitialout-of-planepretiltangleofthe nematic directorn arenecessaryfora linearC(B) dependence inthe lowB limit. Amoredetailedtheoreticalanalysisishowever,neededtojustify thisassumption. Acknowledgments This work was supported by the Slovak Academy of Sciences, in the framework of CEX-NANOFLUID, projects VEGA 0045, the Slo- vak Research and Development Agency under the contract No. APVV-0171-10, the Ministry of Education Agency for Structural Funds of EU in the frame of projects 26110230061, 26220120021 and 26220120033, the Grenoble High Mag- netic Field Laboratory (CRETA), and by the Hungarian Research Fund OTKA K81250. REFERENCES 1. P.G. de Gennes. The Physics of Liquid Crystals. (Clarendon Press, Oxford 1974) 2. V. Fr´eedericksz and V. Zolina. Forces causing the orientation of an anisotropic liquid. Trans. Faraday. Soc., vol. 29 (1933), pp.919-930. 3. F. Brochard and P.G. de Gennes. Theory of magnetic suspensions in liquid crystals. J. Phys. (Paris), vol. 31 (1970), pp.691-708. 4. S.H. Chen and N.M. Amer. Observation of macroscopic collective behavior and newtextureinmagnetically dopedliquidcrystals. Phys. Rev. Lett.,vol.51(1983),pp. 2298-2301. 5. S.V. 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